The Network folder specified is currently mapped using a different user name and password
NET USE does not allow you to connect to the same server with different credentials. This is because it reuses the same relationship to map additional shares on the same server. You can work around this by making windows think it's a different server either by creating DNS aliases for the same NAS with different names or editing the hosts file on the clients.
A better solution would be to change your account structure. Don't create an account on the NAS for each share. Create an account on the NAS for each client and give those accounts permissions to the shares.
If the share is mapped under a different user, you get that message. The mapping is unavailable (to you), and you can't get it to go away. So
- switch to the other user;
- unmap the drive in Windows explorer;
- run "control userpasswords2", Advanced tab, Manage Passwords, Windows Credentials, and remove the login (for good measure);
- reboot;
- try again under your login.